Considerable attention has been directed in recent years to the development of improved single package room temperature vulcanizable (sometimes referred to as "RTV") compositions. Under ideal conditions, these compositions would be stable for an indefinite period when stored in the absence of moisture, and would rapidly cure to a tack-free elastomer upon contact with moisture, including the relatively small proportions of water vapor present in the atmosphere.
Room temperature vulcanizable silicone elastomer compositions curable through a variety of different mechanisms are known in the art. In a typical RTV composition, a major constituent is a silane-based material, such as a polydiorganosiloxane containing polyalkoxysilyl (typically dialkoxyalkylsilyl) end groups. These groups may be formed in situ by incorporating in the RTV composition a silanol-terminated silicone and, as an end capping reagent, a polyalkoxysilyl compound such as methyltrimethoxysilane or dimethyltetramethoxydisilazane, which undergo a reaction to produce the polyalkoxysilyl-terminated species. It is generally more efficient, however, to utilize pre-formed polyalkoxysilyl-terminated materials prepared by the same reaction in the RTV compositions.
In either case, the silane-based material, e.g., the polyalkoxysilyl-terminated species, is capable of being cross-linked by atmospheric moisture in the presence of a suitable metal-containing catalyst, usually an ammonium, titanium or a tin compound. Disclosures of RTV compositions of this type are present in many patents and publications.
Of special interest regarding the present invention are RTV compositions which are curable upon exposure to moisture based on a mechanism utilizing hydrolyzable functional groups contained on the siloxane materials (i.e., a polycondensation reaction). These compositions are generally prepared by combining an organosilandiol, usually a diorganosilanol, with a suitable cross-linking agent, optionally in the presence of standard components such as fillers, plasticizers, sag control agents and the like. An example of such a composition is disclosed in U.S. Pat. No. 4,395,526.
RTV silicone elastomer technology is extremely useful and commercially successful, but slow room temperature curing speeds limit its wider application. Typically, this technology can achieve a depth of cure from top to bottom of only about two millimeters in a twenty-four hour period and three millimeters after a seventy-two hour period. Therefore, means to improve the slow curing speed of RTV silicone elastomers have been actively sought.
One approach to this problem uses methyltriacetoxysilane with a dihydroxy-terminated polydimethylsiloxane under substantially anhydrous conditions. See, for example, U.S. Pat. No. 3,133,891, Ceyzeriat; U.S. Pat. No. 3,035,016, Bruner; and U.S. Pat. No. 3,382,205, Beers. While this approach provides a fast cure and good adhesion, the acetoxysilane cross-linker materials release a strong acid odor and are corrosive to metal substrates.
Alkoxysilane cross-linking agents provide an alternative approach which does not corrode the metal substrates or produce strong unpleasant odors in use (see U.S. Pat. Nos. 4,377,706 and 4,395,042). However, these materials exhibit a relatively slow curing speed.
U.S. Pat. Nos. 3,189,576, 4,323,489, and 3,962,160 generally relate to ketoxime cross-linking agents used in room temperature vulcanizable polysilanol compositions. These ketoxime cross-linkers, however, are also slower than the acetoxy-type materials.
Another solution is proposed in U.S. Pat. No. 3,819,563, where an alkenyloxysilane cross-linker is compounded with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions prior to cure. Upon exposure to the atmosphere, these alkenyloxy cross-linkers provide fast cure speed, no odor and essentially no metal corrosion. Their major drawback is that they are very expensive and not compatible with conventional inorganic fillers which are necessary to achieve good physical properties in the final elastomer product.
It has now been found that when certain formamide-type compounds are used in a catalyzed RTV polycondensation reaction to form silicone elastomeric materials, excellent and improved results are obtained. Specifically, this RTV reaction provides a fast cure together with a good depth of hardening in the final product. The compositions provided for in the present invention can be stably stored under substantially moisture-free conditions and are readily vulcanized to form solid elastomers when exposed to moisture in the atmosphere.
U.S. Pat. No. 4,863,992, Wengrovius et al, issued Sep. 5, 1989, describes the preparation of polyalkoxysilyl-terminated silicones using specific end-capping catalysts which are converted in the reaction mixture to "harmless end products". The catalysts used are amine salts of formic acid (e.g., the salt of di-n-butylamine and formic acid). In the reaction mixture, this material is broken down into butylformamide which was chosen because it is inert in the reaction mixture. There is no suggestion in this patent to use formamide-type materials in an elastomer curing reaction.
U.S. Pat. No. 5,004,793, Nagaoka, et al, issued Apr. 2, 1991, describes an RTV composition using a novel specifically-defined siloxane-amide block copolymer together with a cross-linking agent and an organolin curing catalyst. N,N-dimethylformamide is taught to be useful as a solvent for making the block copolymer, but there is no suggestion to use it in the RTV reaction itself.
U.S. Pat. No. 5,206,328, Okamura, et al, issued Apr. 27, 1993, describes a method for producing organopolysilanes containing an epoxy group. An aprotic polar organic solvent may be used in this reaction; N,N-dimethylformamide is a preferred solvent. There is no suggestion to use this formamide-type material in an RTV reaction.
He, X. W., et al, Eur. Polym. J., 24 (12), 1145 (1988), describes the polycondensation reaction between alpha, omega-dihydroxy polydimethylsiloxane and tri- or tetra-alkoxysiloxane, catalyzed by stannous octoate. There is no suggestion to use formamide-type compounds in this system.